The invention relates to a wiper blade rubber for a windshield wiper, in particular to a windshield wiper of a motor vehicle. It further relates to a process for production of the same.
The wiper blade rubbers or wiper strips within a wiper blade of a windshield wiper serve by way of example for the removal of water droplets on a windshield of a motor vehicle and are moved back and forth by the windshield wipers of a windshield-wiper system. Wiper blade rubbers are usually molded from a hydrophobic extruded or injection-molded starting material, for example from an unvulcanized, synthetic rubber, and then vulcanized. The general requirements placed upon a wiper blade rubber encompass not only high wipe quality but also resistance to abrasion, low frictional resistance, and good slip properties on hydrophilic and hydrophobic surfaces, in order to achieve appropriate running performance of the wiper blade rubber on the various surfaces.
Good wipe qualities are generally achieved using relatively soft wiper blade rubber qualities, but these often exhibit disadvantageous running performance in particular on hydrophobic surfaces, because of a high coefficient of friction. In the prior art, it is known that the surface of the wiper blade rubber can be subjected to post-treatment in order to improve the running performance of the wiper blade rubber. By way of example, the surface is halogen-treated, i.e. chlorination or bromination is carried out. This hardens the surface of the wiper blade rubber, thus reducing the coefficient of friction of the relatively soft rubber material of the wiper blade rubber. Even when frictional resistance is reduced by reducing the coefficient of friction, this is not generally sufficient to obtain a marked improvement in the running performance of the wiper blade rubber. Furthermore, in this way surfaces post-treated often exhibit a low contact angle with respect to water, i.e. are not very hydrophobic, the result being non-ideal wipe quality in particular on hydrophobic surfaces.
It is also known that the running performance of the wiper blade rubber can be improved via surface coating. It is therefore known that it is possible to apply, to the surface of a wiper blade rubber, a layer which usually comprises a fine lubricating powder, examples being pulverulent molybdenum disulfide or graphite. A large number of other materials are known in the prior art for such coatings. DE 10 2005 000 851 A1 describes, for example, a wiper blade with a wiper blade rubber obtained via vulcanization, where a foil or a layer derived from granulated material has been applied to the wiper rubber lip of the wiper blade rubber in order to reduce the coefficient of friction. The coating material can be applied in the form of a jet or spray, or in any other form, to the elastomeric surface.
A further embodiment of the coating of the surface of a wiper blade rubber provides the application of an antifriction coating, with the aim of maximizing the reduction in coefficient of dry friction. DE 101 16 926 A describes antifriction-coating components, antifriction coating, and an appropriate process for the coating of elastomers, where the antifriction-coating components encompass not only a polyurethane and a siloxane but also a polyamide powder, a polyethylene powder, or a solution of a polyamide. The antifriction coating can comprise an isocyanate-based hardener.
The surfaces produced in the case of coating with an antifriction coating, with a foil, or with a layer derived from granulated material are generally coherent, and this means that the coating applied forms a new surface, which completely covers the elastomeric surface situated thereunder. The surface produced by powder coating is generally not coherent, and the elastomeric surface located thereunder is therefore to some extent visible.
Little is generally known about the surfaces of wiper blade rubbers in relation to their performance with respect to water, i.e. their hydrophobicity. The hydrophobicity of materials is a measure of the level of their hydrophobic properties and can be stated in the form of a contact angle which appears between a droplet of liquid, such as water, and the surface of the material, using air as third phase. A surface with a contact angle greater than 90° is termed hydrophobic, at contact angles greater than 120° the surface can be termed highly hydrophobic, and at contact angles greater than 150° it can be termed superhydrophobic. Particularly advantageous wipe qualities of wiper blade rubbers are achieved when the surface is highly hydrophobic, i.e. exhibits a contact angle greater than 120°.
EP 1 249 280 discloses self-cleaning surfaces and a process for their production, where hydrophobic particles with a size in the micrometer range to submicrometer range are applied to a surface and in turn have a fissured structure in the nanometer range. These surfaces exhibit a high level of hydrophobicity, with contact angles greater than 150°.